Ok so, once upon an ARAM game, I, a Nami main, decided to limit test an idea I'd had in my brain for a while.
The idea was this, If supports, specifically Nami, get the most value from either building ability haste for their ults or utility, why don't I try to do both?
So I hastily built my runes and created this abomination you see today!

How this build works is, with both Rapid Fire Canon and Imperial Mandate you can use your E, Tidecaller's Blessing to empower your auto attacks to slow enemies (without your E only your first aa on an enemy unit will slow) and apply Mandate. Thus giving you more agency and play making potential with your Q, Aqua Prison because it should be easier to land after slowing them.
With the item haste runes you will be procing mandate more often and using Redemption more often as well, which is a very important sustain item. The rest of the build is just building upon this idea with the options of Staff of Flowing Water vs Rylai's Staff (which I recommend over SoFW) and Manamune vs Archangel's Staff.